There are three primary structures within the human eye that are essential to vision and subject to age-related damage: the cornea, lens and retina. The retina is a multi-layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned into images. There are two types of photoreceptors in the retina: rods and cones. The retina contains approximately 6 million cones. The cones are contained in the macula, the portion of the retina responsible for central vision. They are most densely packed within the fovea, the very center portion of the macula. Cones function best in bright light and allow us to appreciate color. There are approximately 125 million rods. They are spread throughout the peripheral retina and function best in dim lighting. The rods are responsible for peripheral and night vision. The retina is essential for vision and is easily damaged by prolonged unprotected exposure to visible and near visible light. Light-induced retinal pathologies include cystoid macular oedema, solar retinopathy, ocular melanomas and age-related macular degeneration (ARMD). Light-induced retinal damage is classified as structural, thermal or photochemical and is largely determined by the exposure time, power level and wavelength of light (W. T. Ham. 1983. Journal of Occupational Medicine. 25:2 101-102).
In healthy adults the retina is generally protected from the most severe forms of light-induced damage by the outer eye structures including the cornea and crystalline lens. The cornea is a transparent proteinaceous ocular tissue located before the iris and is the only eye structure exposed directly to the environment. The cornea is essential for protecting the delicate internal structures from damage and facilities the transmission of light through the aqueous media to the crystalline lens. The cornea is the primary light filter and therefore is particularly susceptible to excessive light exposure-related damage including corneo-conjunctival diseases such as pterygium, droplet climatic keratopathy and pinguecula. In the healthy eye, the cornea, in conjunction with the aqueous medium, absorbs, or blocks, wavelengths (nm shall be used hereinafter to denote wavelengths of light in nanometers) in the short ultraviolet (UV)-B and UV-C region (less than ≈320 nm).
The crystalline lens is an accommodating biological lens lying directly behind the iris and cornea and facilitates the convergence of both far and near images onto the retina. The natural crystalline lens blocks near UV radiation (UV-A) (320 nm to 400 nm) from reaching the retina. Therefore, most of the damaging UV-A, -B and -C radiation are prevented from reaching the retina in healthy people with an intact crystalline lens and cornea. Thus in the normal mammalian eye only wavelengths between about 400 nm and 1,400 nm can reach the retina. However, high transmittance levels of blue and violet light (wavelengths from about 390 nm to about 500 nm) has been linked to retinal damage, macular degeneration, retinitis pigmentosa, and night blindness. In addition, blue and violet light tends to be scattered in the atmosphere, especially in haze, fog, rain, and snow, which in part can cause glare, and diminished visual acuity. As the eye ages the crystalline lens begins to take on a yellow tint that absorbs some radiation in the blue and violet wavelength ranges, in addition to the majority of near UV radiation. Thus, the natural crystalline lens protects the eye's delicate retina from near UV light throughout life and subtly yellows with age, thereby increasing the amount of blue and violet light that is absorbed.
The natural crystalline lens is also susceptible to age-related degenerative eye diseases such as cataracts. Cataract is a clouding of the crystalline lens caused by the coagulation of lens proteins within the capsular sac. Many ophthalmologists believe that cataract formation results from a lifetime of oxidative insults to the lens and is exacerbated by smoking, excessive exposure to bight light, obesity and diabetes. Cataracts develop slowly in most people and eventually reach the point where vision is substantially impaired resulting in near to total blindness. In these persons lens removal and replacement with synthetic polymer ophthalmic devices such as an intraocular lens is the preferred means for restoring normal sight. However, once the natural crystalline lens is removed the retina is left unprotected from damaging UV and short wavelength violet light. Thus early synthetic ophthalmic devices were provided with UV absorbing compounds such as benzophenones and benzotriazoles-based UV light absorbers. Moreover, many benzophenones and benzotriazoles are polymerizable and thus can be stably integrated into most modern ophthalmic device compositions including acrylates and hydrophilic hydrogel co-monomers and co-polymers. Ultraviolet light does not play a positive role in human vision. Thus ophthalmic devices having UV absorbing dye concentrations that block virtually all UV light became common-place by the mid 1980s.
In the 1990s ophthalmic devices having violet light absorbing materials such as azo dyes incorporated therein were introduced to approximate the violet light blocking effects of the aging adult natural crystalline lens. For example, U.S. Pat. No. 4,390,676, describes polymethylmethacrylate (PMMA) polymer ophthalmic devices incorporating yellow dyes that selectively absorb UV, violet and blue light radiation up to approximately 450 nm. U.S. Pat. Nos. 5,528,322; 5,543,504; and 5,662,707 are assigned to Alcon Laboratories, Inc. and disclose acrylic-functionalized yellow azo dyes having an inert chemical spacer between the dye and acrylic portions of the molecule. Thus the violet light-absorbing portion of the molecule is protected from undesirable color shifts when polymerized with the lens polymer. Moreover, because the dye is acrylic-functionalized, it is polymerizable with the lens polymer and thus stably incorporated into the ophthalmic device polymer matrix. Similarly, Menicon Co., Ltd. holds U.S. Pat. Nos. 6,277,940 and 6,326,448 both disclosing specific acrylic-modified azo dyes structurally similar to Alcon's. Hoya Corporation owns U.S. Pat. No. 5,374,663 that discloses non-covalently linked yellow dyes including solvent yellow numbers 16, 29 and others incorporated into a PMMA matrix. Moreover, Hoya also owns U.S. Pat. No. 6,310,215 that discloses acrylic-functionalized pyrazolone dyes suitable for use in acrylic and silicone ophthalmic devices.
However, these and other prior art ophthalmic devices have the violet blocking dyes evenly distributed throughout the ophthalmic device material at concentrations that simulate the natural yellow color of the 53 year-old individual's crystalline lens. Consequently, all light and images are filtered through a yellow color before being projected on the retina. For activities that rely on acute photopic sensitivity (day light visual conditions) this may be desirable. For example, people who engage in certain day time outdoor sports or activities including skiers, baseball players, football players, pilots, and boaters are exposed to high levels of ultraviolet, violet, and visible light radiation which can affect visual acuity required in such activities. Drivers of motor vehicles also have specific needs in terms of reducing glare and enhancing visual acuity under bright, sunlit driving conditions and reducing headlight glare at night.
However, unlike UV radiation, the violet light spectrum (440 nm to about 500 nm) are important for maintaining optimal visual acuity, especially scotopic (night) vision. Thus ophthalmic devices containing dyes that block significant amounts of violet light over the majority of the violet light spectrum can adversely affect scotopic vision. This is an especially acute problem in older adults that naturally suffer declining scotopic vision and reduced pupil dilation. Consequently, an ophthalmic device is needed that balances the need for reducing the possible damaging effects of blue and violet light exposure against the need to maintain good scotopic vision.
Therefore, it is an objective of the present invention to provide an ophthalmic device having a highly selective (abrupt) violet light transmissive filter incorporated therein that protects against radiation in the violet waveband and more damaging portions of the blue waveband, thus providing improved scotopic vision when compared to prior art devices.